1. Field of the Invention
The present invention relates to a PM stepping motor and more particularly to a pole tooth structure of a PM stepping motor.
2. Description of the Related Art
A PM (permanent magnet) stepping motor provides a high rotational precision and therefore is used for various kinds of actuators. The PM stepping motor (hereinafter referred to simply as stepping motor) generally includes a circular cylindrical rotor magnet which has a plurality of magnetic poles magnetized circumferentially thereon and a pair of stator units which are axially coupled to each other and are disposed around the rotor magnet. Each of the stator units is structured such that two yokes (specifically inner and outer yokes) each having a plurality of pole teeth arranged circumferentially in a comb-like manner are disposed to axially oppose each other with their respective pole teeth meshing with each other and such that an exciter coil (stator coil) is attached around the intermeshing pole teeth.
The stepping motor structured as described above, when used as an actuator in, for example, an automobile air conditioner, is requested to be less-vibration (lower-noise). A stepping motor with suppressed vibration for use as an actuator in an automobile air conditioner is disclosed in, for example, Japanese Patent Application Laid-Open No. 2006-311708.
Referring to FIG. 6, in the stepping motor disclosed in Japanese Patent Application Laid-Open No. 2006-311708, a stator unit includes a yoke including a magnetic pole tooth (pole tooth) 144 which has a geometry defining upright sides 144a and 144b extending from a base portion 143 of the yoke so as to be perpendicular to the base portion 143, a top end side 144c, and slant sides 144d and 144e connecting respectively between the distal end of the upright side 144a and one end of the top end side 144c and between the distal end of the upright side 144b and the other of the top end side 144c, wherein the pole tooth 144 has a width A at the top end side 144c, a width B at the base portion 143 of the yoke, and a height H that is an axial direction distance measuring from the base portion 143 to the top end side 144c, and wherein the ratio of the width A to the width B is 65% or more and at the same time the ratio of the height H to the width B ranges from 90% to 110%.
When the geometry of the pole tooth 144 is defined as described above, the content of magnetic flux interlinked with the stator coil is adapted to be changed so that the waveform of induced voltage of the stator coil can be adjusted to thereby reduce generation of torque which triggers vibration. As a result, the vibration caused when the motor runs can be reliably suppressed and a very quiet stepping motor can be provided without taking conventional countermeasures such as usage of additional components in en electronic circuit and provision of an additional space for such components.
The stepping motor disclosed in Japanese Patent Application Laid-Open No. 2006-311708, however, is involved with dimensional constraints in the pole tooth 144, specifically such that the ratio of the width A (at the top end side 144c) to the width B (at the base portion 143 of the yoke) and the ratio of the height H to the width B are subject to constraint. That is to say, since the ratio of the height H to the width B is required to range from 90% to 110%, the pole tooth 144 has a substantially square shape in plan view, and consequently the height H of the pole tooth 144 cannot be arbitrarily increased when the width B is set to a predetermined value.
Under the circumstances described above, the upper limit of the surface area of the pole tooth 144, with which the magnetic flux from the stator coil or the rotor magnet is interlinked, is limited, and therefore the starting torque cannot be increased to exceed a certain value. If the width B at the base portion 143 of the yoke is increased in order to allow the height H to increase, then the diameter of the yoke must be inevitably increased, thus forcing the diameter of the motor to increase.
Also, in the pole tooth 144 of the stepping motor disclosed in Japanese Patent Application Laid-Open No. 2006-311708, the ratio of the width A at the top end side 144c to the width B at the base portion 143 must be 65% or more. Consequently, since the pole tooth 144 does not include a portion whose circumferential dimension (width) is small compared with a triangle-shaped pole tooth generally employed, the precision in determination of the rotational position of the motor may be deteriorated.
Further, when the yoke with the pole tooth 144 is produced by punching a planar magnetic material (refer to Paragraph [0016] of Japanese Patent Application Laid-Open No. 2006-311708), more waste is generated compared when the yoke with a triangle-shaped pole tooth is produced by punching.
Moreover, generally, detent torque is to be lowered in order to suppress vibration, but when the stepping motor is used in, for example, an automobile air conditioner, detent torque must be set to a reasonably high level of value for preventing it from happening that the stepping motor at a stop is caused to rotate due to wind pressure.